The tremendous potential for gene therapy has been demonstrated in the last decade for many different diseases (reviewed in Ghosh, et al., Gene therapy for monogenic disorders of the bone marrow. Br J Haematol. (2015)). The simplest strategy proposed to distribute gene therapy involves direct in vivo gene modification. Efforts to achieve gene transfer in vivo in small and large animal models are underway (Burtner, et al., Blood 123, 3578-3584 (2014); Kay, et al., Science 262, 117-119 (1993); Ponder, et al., Proc Natl Acad Sci USA 99, 13102-13107 (2002); Ting-De Ravin, et al., Blood 107, 3091-3097 (2006); Frecha, et al., Blood 119, 1139-1150 (2012)), but it will likely be some time before this approach meets current safety and efficacy standards to permit clinical testing in subjects. Major hurdles include stringent evaluation of gene transfer to non-target cells balanced with achieving sufficient therapeutic gene transfer levels.
Ex vivo mediated gene transfer into target cells is one clinically applied method for gene therapy demonstrating efficacy to date. In stem cells, this approach allows for subsequent production of all blood cell types harboring the therapeutic genes for the lifetime of the patient. The isolation and genetic modification of CD34+ stem cells ex vivo provides two major benefits: elimination of gene transfer to non-target cells and reduced average amount of genetic modifiers, e.g. nucleic acid carriers, which in turn reduces costs associated with carrier production. To manufacture these products within current regulatory guidelines, however, typically requires complex centralized facilities adhering to current Good Manufacturing Practices (cGMP).